The Syrian army battled Monday alongside Hezbollah militants to reclaim a rebel-held city, amplifying pressure on President Barack Obama to find a way to alter the course of events in the widening civil war.

The bloody battle over the city of Qusayr, near the Lebanese border, has the potential to transform Syria's conflict, say fighters, diplomats and analysts. A government victory there could give the regime of President Bashar al-Assad a corridor of territory connecting Damascus to Syria's pro-Assad coastline and to Lebanese territory controlled by Iran-backed Hezbollah. This would split rebel forces into fragmented strongholds.

It is too soon to say whether the battle will shape up as a turning point in the largely stalemated two-year conflict, said U.S. military and intelligence officials. But the fierce fighting and the regime's use of airstrikes and artillery bombardment ignited new criticism of the Obama administration's policy of responding to the war with international diplomacy and humanitarian aid.

ENLARGE

On Tuesday, senators are due to debate a bipartisan bill allowing the administration to provide lethal assistance to Syrian rebels, a move likely to pick up momentum from reports of regime gains in Qusayr.

"The entire paradigm has shifted" in Syria's conflict, a Western diplomat said, describing the government's push into Qusayr as the latest in a string of "confident, defiant and strategic moves."

The battle also illustrates the regional spread and increasingly sectarian cast of Syria's civil war—with the Shiite Hezbollah now in armed conflict with a largely Sunni opposition, and the Syrian regime, guided by Iran, accusing Israel of abetting rebels.

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The Assad government's two-day offensive, launched late Saturday, comes after weeks of a tightening siege and as international leaders plan talks in Geneva as soon as next month. Many analysts believe Mr. Assad is using maximum military force in an effort to improve his standing at those talks.

"You want to negotiate from a position of strength," said Aram Nerguizian, of the Center for Strategic and International Studies. "If Assad has Qusayr…he will be in a better negotiating position."

A victory in Qusayr would allow Mr. Assad's army and its allies to continue to purge rebel strongholds in the country's center and west, said military analysts close to Hezbollah and supporters of the Syrian regime. It would isolate rebels in the north—who control most of the territory along the border with Turkey—from rebels in the south, who are planning an upward march on Damascus.

Previously

Seven people, including three rebels, were killed in the fighting Monday, according to activist estimates, following at least 48 deaths reported on the rebel side Sunday. At least two dozen Hezbollah fighters have been killed and 50 more injured in two days of fighting, according to accounts from rebels in Qusayr, residents of Hezbollah strongholds in Lebanon and a Lebanese security official.

Syrian state media said the military made major headway Monday, taking over the entire eastern part of Qusayr and continuing the pursuit of what it labeled as "terrorists" in the north and south.

Activists and rebels in Qusayr disputed the government had taken any large chunk of the city Monday, saying that direct attacks had eased Monday and that they were attempting to evacuate injured rebels out of the town amid the government siege.

Later Monday, rebels said they had killed Hezbollah fighters, as well as regime troops, in an ambush involving booby-trapped cars in the city's east. The number couldn't immediately be confirmed.

"We are fighting Iran, Russia, the party of the devil and the regime," a Qusayr rebel commander, identified as Abu Ali, said after the attack in a video interview posted online by an activist network called the Qusayr Media Center, making a play on the name Hezbollah, or Party of God. "The whole world has left the innocent Syrian people on their own."

A government capture of Qusayr could mark the start of a wider campaign to reconquer territory at a time when rebels are facing some of their stiffest challenges yet in territories they control. "This would make rebel advancements around Damascus nearly impossible," a fighter on the outskirts of Qusayr said.

The fight between the two camps has become increasingly unequal, diplomats and analysts said.

"This is really tipping the balance in favor of the pro-regime camp in very visible ways," an analyst in Damascus said.

"This is a struggle that poses, on one side, an axis that is cohesive, determined and ruthless," the analyst said. "On the other side, you have a group of players that are getting distracted by secondary issues, don't agree on much, and are losing momentum."

The regime has in recent weeks gained significant ground in Damascus and the center. Backed by Iranian and Hezbollah military advisers, the regime is focused on cutting off rebel supply lines into the city from the north and south, and breaking the vast agricultural suburbs of Damascus into smaller theaters of operation that the military can easily besiege, say regime officials.

The prospect of a rebel loss in Qusayr prompted condemnation from the Obama administration over the Assad regime's use of force and the deployment of Hezbollah as a fighting force. On Monday, Mr. Obama called Lebanese President Michel Sleiman and complained, pointing out that the Syrian involvement of Hezbollah—which is a partner in Lebanon's government—was contrary to Lebanon's policy of disassociation from the conflict.

Mr. Sleiman repeatedly has urged Lebanon's dueling sectarian factions to stick by the state's policy. But Hezbollah has backed the Syrian government and the Western-backed, Sunni-led alliance in Lebanon known as March 14 has supported the rebels with funds, arms and fighters.

Despite an erosion in the rebels' fortunes, the Obama administration held fast to plans to await an international conference for which expectations are low as world leaders plan to discuss the conflict and possible political solutions.

The conference is still being organized, with key questions unanswered, including whether Iran, a key ally of Mr. Assad, will participate. Secretary of State John Kerry was flying to the Middle East on Monday, planning to attend a meeting Wednesday in Jordan of a group of leaders seeking the removal of Mr. Assad.

But as the Qusayr battle continues, the White House faces increasing scrutiny.

"This is a significant and dramatic indicator of the Obama administration's passivity," said Sen. John McCain (R., Ariz.). "For this administration to sit idly by and watch these events unfold, is writing a shameful chapter in American history. The longer this conflict goes on, the more unstable the region will be."

Mr. McCain criticized the administration's reliance on the Geneva conference as "optimistic to the point of fantasy," adding that each time the U.S. has upped its assistance to rebels, Iran and Russia have responded with new aid to Mr. Assad.

"Isn't it Orwellian that we are relying on a conference headed by the United States and Russia, while Russia is shipping sophisticated missiles to Bashar al-Assad?" he asked.

"We are doing everything we can to end this violence," said Patrick Ventrell, a State Department spokesman. "We know how hard the road ahead is to get to a negotiated political settlement, but that's the best way to end the violence."

The view of the fighting in Qusayr is complicated by overlapping objectives of those fighting, according to people on the ground, analysts and government officials.

Mr. Assad has an interest in shifting the strategic balance in his war-torn country, and can produce the appearance of big victories by pushing rebels out of long-held areas.

At the same time, U.S. military officials believe the battle for Qusayr shows that Mr. Assad and his allies are trying to solidify their control over a key transit point, which would deal a blow to the opposition. Hezbollah would benefit not only by supporting its ally, Mr. Assad, but also by keeping one of its own key supply routes—from Iran, and through Syria—from rebel hands.

But the location of Qusayr, between Damascus and Syria's coastal Latakia province, also suggests the location may be important to future plans by Mr. Assad. U.S. intelligence agencies think the offensive may be part of an effort by Mr. Assad to clear an approach to Latakia, which could become a haven for Alawites, the Shiite-linked minority sect of the Assad regime.

"We don't think it means a strategic shift in the dynamics of the conflict," a senior U.S. official said of the battle. "It may mean there's a connection between this action and building towards an Alawite enclave."

U.S. officials are increasingly concerned about Syria's breakup along sectarian lines, not only because of the potential breakup of the country into hostile territories, but because of heightened chances for the conflict to spill into neighboring countries.

The U.S. is particularly concerned about possible spillover of fighting into Jordan and Iraq. Defense Secretary Chuck Hagel has asked Pentagon war planners to draw up contingency plans for these and other "worst case scenarios," officials have said.

—Rima Abushakra in Beirut and Adam Entous in Washington contributed to this article.

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